It's likely you're not inputting the directory properly. The command you enter in should be something like: find /Users/me/mydir/ -name \*.png […], with no angled brackets, and if you have any spaces in the directory path, be sure to put it in quotes, like so: '/Users/me/my directory/'.

It's also worth noting that the command will act on any PNGs not just within the directory you give it, but also any subdirectories.

An alternate (and slightly more concise) version that just acts on the current directory would be: